Usually, an organic electroluminescent (organic EL) display device which can be used in various display apparatuses is produced by using a process similar to a semiconductor production process, and forming light-emitting cells each having an organic electroluminescent portion on a substrate such as a semiconductor. The light-emitting cells are used for displaying pixels constituting an image or the like to be displayed, respectively.
The organic EL uses a phenomenon called injection electroluminescence in which light is emitted by recombination of an electron-hole pair. Since the luminescence principle is similar to that of an LED (Light-Emitting Diode), an organic EL portion is also called an OLED (Organic Light-Emitting Diode).
In order to surely control lighting/extinction of each of many light-emitting cells which are arranged two-dimensionally, usually, an active matrix drive system in which an independent active drive element such as a TFT (Thin Film Transistor) is disposed for each of the cells is used.
In the case of an organic electroluminescent display device, a circuit which is configured as shown in, for example, FIG. 8 of JP-A-2005-300786 (corresponding to US 2005/0225253 A1) is formed for each cell. Namely, a driving transistor (80) which is connected in series to an organic EL element (70) is disposed in order to control energization of the element, and a capacitor for holding a signal and a selection transistor (10) for switching the signal are connected to the input of the driving transistor.
At a timing when a signal which is to be displayed in the cell appears, the selection transistor is temporarily turned on, and the necessary signal is held by the capacitor. Therefore, the driving transistor for the cell supplies a current corresponding to the input signal to the organic EL element, so that the luminous intensity of the organic EL element is controlled by the current.
In an organic EL display device in which many light-emitting cells are arranged, it is important to increase the aperture ratio of each cell. More specifically, each cell tends to have a small area, and therefore a sufficient luminous intensity cannot be obtained and a clear display is not enabled unless the area ratio of the luminous region to the cell is increased as far as possible. Actually, when transistors or the like of a circuit for driving cells are increased in size, light is blocked by the transistors or the like, so that the aperture ratio of each cell is lowered and the luminous intensity is reduced.
In the prior art disclosed in JP-A-2005-300786 (corresponding to US 2005/0225253 A1), in order to enable the channel of the driving transistor to be shortened, therefore, the carrier mobility of the driving transistor is made lower than that of the selection transistor. Specifically, a silicon-based semiconductor is used as the active layers (regions where the channel is formed) of the transistors, and their carrier mobilities can be changed depending on the difference of their grain sizes.
JP-A-2006-186319 (corresponding to US 2006/0113549 A1) discloses a luminescence device which is configured by using an amorphous oxide semiconductor in the active layer of a transistor.